We define a delta payload that can be returned from a(n opaque) delta link, as well as other service-defined mechanisms.

However, we don't have a way to define a custom function (or action) that can return a delta payload, and there are many places where a service may want to describe the results of a custom function (or action) using a delta payload.

Similarly, one can imagine using the delta payload as a payload to a custom action.

How can users define functions/actions that can return (or take) delta payloads.